clipper/backend/media/get_segment.go

package media

import (
	"bytes"
	"context"
	"fmt"
	"io"
	"os/exec"
	"strconv"
	"sync"
)

// CommandFunc is a function that builds an *exec.Cmd from a context, name and
// args.
type CommandFunc func(ctx context.Context, name string, arg ...string) *exec.Cmd

// AudioFormat represents an abstract audio format, e.g. MP3 or WAV.
type AudioFormat int

const (
	AudioFormatWAV AudioFormat = iota
	AudioFormatMP3
)

// String implements fmt.Stringer.
func (f AudioFormat) String() string {
	switch f {
	case AudioFormatWAV:
		return "wav"
	case AudioFormatMP3:
		return "mp3"
	default:
		panic("unknown audio format")
	}
}

// AudioSegmentProgress represents a progress update for an AudioSegmentStream,
// and contains a byte slice of audio data and indication of the approximate
// progress.
type AudioSegmentProgress struct {
	PercentComplete float32
	Data            []byte
}

// AudioSegmentStream is a stream of AudioSegmentProgress structs.
type AudioSegmentStream struct {
	progressChan chan AudioSegmentProgress
	errorChan    chan error
}

// send publishes a new partial segment and progress update to the strean.
func (s *AudioSegmentStream) send(p []byte, percentComplete float32) {
	s.progressChan <- AudioSegmentProgress{
		Data:            p,
		PercentComplete: percentComplete,
	}
}

// close signals the successful end of the stream of data.
func (s *AudioSegmentStream) close() {
	close(s.progressChan)
}

// closeWithError signals the unsuccessful end of a stream of data.
func (s *AudioSegmentStream) closeWithError(err error) {
	s.errorChan <- err
}

// audioSegmentGetter gets an audio segment and streams it to the caller.
type audioSegmentGetter struct {
	mu                       sync.Mutex
	commandFunc              CommandFunc
	rawAudio                 io.ReadCloser
	channels                 int32
	outFormat                AudioFormat
	stream                   *AudioSegmentStream
	bytesRead, bytesExpected int64
}

// newAudioSegmentGetter returns a new audioSegmentGetter. The io.ReadCloser
// will be consumed and closed by the getAudioSegment() function.
func newAudioSegmentGetter(commandFunc CommandFunc, rawAudio io.ReadCloser, channels int32, bytesExpected int64, outFormat AudioFormat) *audioSegmentGetter {
	return &audioSegmentGetter{
		commandFunc:   commandFunc,
		rawAudio:      rawAudio,
		channels:      channels,
		bytesExpected: bytesExpected,
		outFormat:     outFormat,
		stream: &AudioSegmentStream{
			progressChan: make(chan AudioSegmentProgress),
			errorChan:    make(chan error, 1),
		},
	}
}

// Read implements io.Reader and is consumed by the stdin of the FFMPEG
// command. It is called from a separate goroutine to Write().
func (s *audioSegmentGetter) Read(p []byte) (int, error) {
	n, err := s.rawAudio.Read(p)

	s.mu.Lock()
	defer s.mu.Unlock()
	s.bytesRead += int64(n)

	return n, err
}

// Write implements io.Writer and consumes the stdout of the FFMPEG command. It
// is called from a separate goroutine to Read().
func (s *audioSegmentGetter) Write(p []byte) (int, error) {
	s.stream.send(p, s.percentComplete())
	return len(p), nil
}

func (s *audioSegmentGetter) percentComplete() float32 {
	s.mu.Lock()
	defer s.mu.Unlock()

	return (float32(s.bytesRead) / float32(s.bytesExpected)) * 100
}

// Next implements AudioSegmentStream.
func (s *AudioSegmentStream) Next(ctx context.Context) (AudioSegmentProgress, error) {
	select {
	case progress, ok := <-s.progressChan:
		if !ok {
			return AudioSegmentProgress{}, io.EOF
		}
		return progress, nil
	case err := <-s.errorChan:
		return AudioSegmentProgress{}, err
	case <-ctx.Done():
		return AudioSegmentProgress{}, ctx.Err()
	}
}

func (s *audioSegmentGetter) getAudioSegment(ctx context.Context) {
	defer s.rawAudio.Close()

	var stdErr bytes.Buffer
	cmd := s.commandFunc(ctx, "ffmpeg", "-hide_banner", "-loglevel", "error", "-f", "s16le", "-ac", itoa(int(s.channels)), "-ar", itoa(rawAudioSampleRate), "-i", "-", "-f", s.outFormat.String(), "-")
	cmd.Stderr = &stdErr
	cmd.Stdin = s
	cmd.Stdout = s

	if err := cmd.Start(); err != nil {
		s.stream.closeWithError(fmt.Errorf("error starting command: %v, output: %s", err, stdErr.String()))
		return
	}

	if err := cmd.Wait(); err != nil {
		s.stream.closeWithError(fmt.Errorf("error waiting for ffmpeg: %v, output: %s", err, stdErr.String()))
		return
	}

	s.stream.close()
}

func itoa(i int) string { return strconv.Itoa(i) }